The histocompatibility-2 (H-2) complex of the mouse consists of a series of closely linked loci on chromosome 17 which control protein and cell membrane antigens involved in a number of critical functions in cellular immunity. This proposal suggests two programs for expanding our knowledge of the genetic organization of the H-2 complex and for dissecting and defining the genetic control of the many important immunological phenomena associated with this gene complex. The first approach involves the development of a large number of intra-H-2 recombinants from a single heterozygote combination (H-2b/H-2ti) to provide two series of recombinant strains which are genetically identical except for a portion of chromosome within the H-2 complex. The second approach involves an attempt to increase our catalogue of H-2 linked Ir genes by investigating the immunogenicity of a new class of antigens of restricted antigenic heterogeneity. This class of antigen is produced by the covalent attachment of polyethylene glycol to a complex antigen, e.g., bovine liver catalase, to reduce the number of effective antigenic determinants.